c - 即使没有声明也使用全局定义

Question

在这段代码中:

#include<stdio.h>

int var=100;

int main()
{
    extern int var;        //Declaration not Definition
    printf("%d\n",var);
    return 0;
}

100 被打印出来，这并没有什么不正常，但是当从 ma​​in() 中删除声明时，即使这样也会使用全局定义。这是怎么回事？这取自 K&R，它说:

The (global) variable must also be declared in each function that wants to access it.

Answer 1

对于在同一文件或模块中声明的变量，无需包含 extern 关键字。全局变量对 main 可见，因为它具有全局范围(即它对文件/模块中的所有函数可见)。

To clarify, using extern int x; tells the compiler that an object of type int called x exists somewhere. It's not the compilers job to know where it exists, it just needs to know the type and name so it knows how to use it. Once all of the source files have been compiled, the linker will resolve all of the references of x to the one definition that it finds in one of the compiled source files.

Source